1. Technical Field of the Invention
The present invention relates generally to a power converter for an electric rotating machine which is equipped with a switching circuit and a controller, and more particular to such a power converter designed to ensure improved reliability in operation when the power converter is in a transient state or subjected to an unexpected change.
2. Background Art
Japanese Patent First Publication No. 2004-7964, assigned to the same assignee as that of this application, discloses an inverter for a three-phase motor generator and teaches a technique of setting an off-time of a MOS transistor for one of phases which is the time when the MOS transistor is to be turned off (i.e., after advance of 120° in electrical angle) based on an on-time of a MOS transistor for another phase which is the time when the MOS transistor is to be turned off and regarding the off-timing by a maximum of 60° in electrical angle based on the speed of the motor generator or a duty cycle of a pulse signal for switches of a field current controller.
The above system changes an on-duration of the MOS transistor between 120° to 180° in electrical angle. The range of this change is determined as a function of the speed of the motor generator or the duty cycle of the switches of the field current controller in the feedforward mode. This control, however, has a difficulty in setting the on-duration of the MOS transistor accurately when the system is in a transient state or dealing with an unexpected change such as an aged deterioration in characteristic of the system or a change in ambient temperature, which results in unwanted shift in the off-time. For example, when the system is in the transient state, the off-time may be changed due to effects of a time constant of an excited coil regardless of the duty cycle of the switches of the field current controller. When the unexpected change has occurred, it also results in unwanted shift in the off-time. The advancement of the off-time usually leads to an increased loss in rectification in the inverter. Conversely, the delay of the off-time results in backflow of current from a power supply to windings of the motor generator. In other words, the duration in which current freewheels to the MOS transistors working as rectifying devices is susceptible to change, thus resulting in deterioration of reliability in operation of the system.